IEEE 802.1 P,Q - QoS on the MAC level
24.4.1999
Niclas Ek
Department of Electrical Engineering
Helsinki University of Technology
Abstract
Purpose of this research paper is to study use of protocols described in IEEE (Institute of Electrical and Electronics Engineers) standards 802.1P and 802.1Q as QoS (Quality of Service) protocols on MAC (Medium Access Control) level. First of all, both standards will be represented, as well as some other related standards as well as some proposals for standards. In this paper, a discussion of future of thise standards will be represented. As well will relations to other QoS, ToS (Type of Service) and CoS (Class of Service) standards. Some practical studies of several implementations of thise protocols will carried out in this paper.
Contents
1. Introduction
2. VLANs
2.1 802.1Q
2.2 802.1D
2.3 802.1p
3. QoS
3.1What is QoS
4. 802.1p and QoS
4.1 802.1p - a real QoS protocol?
4.2 Standard is ready - how about implementations
5. Other QoS protocols
5.1 Integrated Serveces
5.2 Resource ReSerVation Protocol (RSVP)
5.3 Differentiated Services
5.3 Multi-Protocol Label Switching (MPLS)
5.4 Constraint Based Routing
6. Future of MAC level QoS protocols
7. References
8. Further Information
1. Introduction
This document specifies use of the IEEE standards 802.1P,Q as Quality of Service protocol on MAC level. Today´s Internet provides only Best Effort Service. Since ethernet is the most spread LAN access technology, importance of providing it a quality of service mechanism ought not to be neglected.
In future ethernet technology will be used as WAN technology, not only as LAN technology. Due to rapidly increasing use Internet throug Public Switched Telecommunication Network (PSTN), Telephone Companies are forced to implement IP-based networks as their PSTN backbones. A network like this whitout any Quality of Service mechanisms would be disastrous. Just imagine yourself trying to get an emergency call throug while others just surf the Internet.
First of all, relationships between IEEE standards 802.1p, 802.1D and 802.1Q have to clarified. IEEE standard 802.1p is a part of the IEEE standard 802.1D. The 802.1p standard covers traffic class expediting and dynamic multicast filtering part of media access control (MAC) bridges, which is known as the IEEE standard 802.1D. [1] IEEE standard 802.1Q is part of the IEEE standard 802.1D, defining an architecture for Virtual Bridged LANs and services provided in Virtual Bridged LANs.
Secondly, expression Quality of Service have to defined. According to the IEEE standard 802.1p, following parameters are essensial for providing QoS. [2]
· 1) Service Availability
· 2) Frame loss
· 3) Frame missorder
· 4) Frame duplication
· 5) The transit delay experienced by frames
· 6) Frame lifetime
· 7) The undetected frame error rate
· 8) Maximum service data unit size supported
· 9) User priority
· 10) Throughput
Thise QoS parameters will be discussed in details in a later section.
Related standards, like RSVP, Differentiated Services and MPLS will be studied. Emphasis will be paid for status of QoS standards, status of appling thise standards in practice. The strong and the weak points of thise standards will be determinated. The future of providing QoS in switched etherhets will be discussed.
2. VLANs
2.1 802.1Q
The IEEE 802.1Q standard defines an architecture for Virtual Bridged LANs, the services provided in Virtual Bridged LANs and the protocols and algorithms involved in the provision of those servises.[1]
No Quality of Service mechanisms are defined in this standard, but an important requirement for providing QoS is included in this standard, e.g. abitity to regenerate user priority of received frames using priority information contained in the frame and the User Priority Regeneration Table for the reception Port. [1]
2.2 802.1D
The updated IEEE 802.1D: ISO/IEC 15802-3 (MAC Bridges) standard covers all parts of the Traffic Class Expediting and Dynamic Multicast Filtering described in the IEEE 802.1p standard. All parts of the IEEE 802.1p standard are merged with old versions of IEEE 802.1D standard. All QoS issues will be discussed in IEEE 802.1p section.
2.3 802.1p
IEEE 802.1p standard, Traffic class expediting and dynamic multicast filtering. Descibes important methods for providing QoS at MAC level.
Quality of Service maintenance
Service availability
Service availability is measured as ratio between MAC service is unavailable and available. In order to increase service availability automatic reconfiguration of the Bridged Local Area Network ought to be adopted.
Frame loss
The Mac Service does not provide a guaranteed delivery of Service Data Units, but the probability is high. Frame loss might occur due to:
· a) Frame corruption in physical layer
· b) Frame is discarded by bridge due to:
o 1) frame has reached maximum lifetime
o 2) exhaustion of internal buffering capacity
o 3) the size of the service data unit frame is carring is too large for LAN it is heading to.
o 4) Bridged Local Area Network is forced to discard frames in order to maintain other QoS aspects.
Frame misordering
The MAC Service does not permit reordering frames with in the same user_priority for a source and destination address pair.
Frame duplication
The MAC Service does not permit duplicating frames.
Transit delay
Frame transit delay is the elapsed time between an MA_UNITDATA.request and corresponding MA_UNITDATA.indication on a successfull transfer.
Frame lifetime
If the maximum deleys a frame has imposed by all the bridges in the Bridged Local Area Network exeeds the desired maximum frame lifetime, the frame ought to be discarded.
Undetected frame error rate
By using FCS calculations for each frame, the undetected frame error rate is very low.
Maximum Service Data Unit Size
The Maximum Service Data Unit Size is dependent on the access media used. A bridge between two LANs has the Maximum Service Data Unit Size of the that has the smaller one.
Priority
MAC Service counts user_priority as a QoS parameter.
Throughput
The total throughput of a Bridged Loval Area Network can greater than one of its equivalet LANs.
Support by ISO/IEC DIS 8802-12 (Demand Priority)
ISO/IEC DIS 8802-12 priority value "normal" maps to user_priority 0 and value "high" maps to user_priority 4. On frame transmission User_priority values 0 through 3 map to "normal" and values 4 through 7 map to "high". access_priority values 0 through 3 map to
"normal" and access_priority values 4 through 7 map to "high".
Regenerating user priority
Based on priority information contained in the frame and the User Priority Regeneration Table for the reception port user_priority will be regenerated for each received frame. Each port has its own User Priority Regeneration Table and each table has eight entries, one for every possible value of user_priority. Mapping explaned in Table 1.
Regenerated
User Priority / Range
0 / 0 / 0-7
1 / 1 / 0-7
2 / 2 / 0-7
3 / 3 / 0-7
4 / 4 / 0-7
5 / 5 / 0-7
6 / 6 / 0-7
7 / 7 / 0-7
Table 1. User Priority Regeneration
Queuing frames
The Forwarding prosess may provide one or more transmission queues for each Bridge Port. It provides storage for queued frames, waits for a oppornity to submit these transmissions. Frames will be assigned to each queue according their user_priority.
The transmission order for a single swicth port is:
· 1) unicast frames with given user_prority for some combination of destination_address and source_address.
· 2) multicast frames with given user_prority for some destination_address.
Frames are assigned to their proper storage queues prior their user_priority using a traffic class table ( table 2. ).
The default algorithm for frame selection for transmission:
· For each port, frames will be selected according to traffic classes supported for that port. Frames will be transmitted only if higher order queues are empty during selection prosess.
· For each queue, the order in which frames are transmitted correspond guidelines described in previous section.
1 / 2 / 3 / 4 / 5 / 6 / 7 / 8
User
Priority / 0 / 0 / 0 / 0 / 1 / 1 / 1 / 1 / 2
1 / 0 / 0 / 0 / 0 / 0 / 0 / 0 / 0
2 / 0 / 0 / 0 / 0 / 0 / 0 / 0 / 1
3 / 0 / 0 / 0 / 1 / 1 / 2 / 2 / 3
4 / 0 / 1 / 1 / 2 / 2 / 3 / 3 / 4
5 / 0 / 1 / 1 / 2 / 3 / 4 / 4 / 5
6 / 0 / 1 / 2 / 3 / 4 / 5 / 5 / 6
7 / 0 / 1 / 2 / 3 / 4 / 5 / 6 / 7
Table 2. Recommended user priority to traffic class mappings
Traffic Types
· a) Network Control; High requirement to get through to maintain and support the network infrastructure
· b) Voice; less than 10 millisecond delay
· c) Video; less then 100 millisecond delay
· d) Controlled Load; some important application
· e) Excellent Effort; Best Effort for important users
· f) Best Effort; ordinary LAN priority
· g) Background; bulk transfers, games etc.
Number of Queues / Traffic Types1 / a, b, c, d,
e, f and g
2 / a, b, c and d
e, f and g
3 / a and b
c and d
e, f and g
4 / a and b
c and d
e and f
g
5 / a and b
c
d
e and f
g
6 / a and b
c
d
e
f
g
7 / a
b
c
d
e
f
g
Table 3. Traffic Type to Traffic Class mapping
user_priority / Traffic Type1 / Background
2 / Spare
0 / Best Effort
3 / Excellent Effort
4 / Controlled Load
5 / Video
6 / Voice
7 / Network Control
Table 4. User Priority to Traffic Class mapping.[2]
3. QoS
3.1 What is QoS
It is not easy to find an ultimate definition for Quality of Service. Each service has its own definition for QoS and each service can be described by its QoS characteristics. Most of the QoS definitions available are ITU-T's standards for ISDN. For datacommunication network performance, QoS characteristics are bandwidth, delay, and reliability. QoS characteristics for performance include:
· Bandwidth: Peak Data Rate (PDR), Sustained Data Rate (SDR), Minimum Data Rate (MDR).
· Delay: End-to-End or Round-Trip Delay, Delay Variation (Jitter).
· Reliability: Availability (as % Uptime), Mean Time Between Failures/Mean Time To Repair (MTBF/MTTR), Errors and Packet Loss.
These as well as many other QoS characteristics could be used to define services in the network. Wheter thise are suitable for determinating QoS in some network depends on the applications and the services used in that network or networks. [3]
4. 802.1p and QoS
4.1 802.1p - a real QoS protocol?
IEEE 802.1p standard has a prioritation scheme, which is in fact quite good. Lower priority level packet are not sent, if there is pakets in queued in higher level queues.
IEEE 802.1p decibes no addmission control protocols. It would be possible to give Network Control priority to all packets and the network would be easily cognested. Even microsoft has found it out. But none of Microsofts operating systems denies it, that is, in Microsofts point of view, network interface card drivers duty. We can only hope, that network interface card manufacturers make good drivers. [13]
IEEE 802.1p standard itself does not limit the amount of resouces one application uses, but many implementations do. A mechanism to negotiate a guaranteed QoS for each appliation, end-to-end, according to the network policy maintained by local network administrators would be an improment with a high priority.
One major shortage, e.i. interoperability has been fixed by the SBM RFC. This RFC deals how to deliver end-to-end QoS to and from etnernet to other networks. The SBM RFC will be discussed in more depth in section 4.3.
4.2 Standard is ready - how about implementations
The IEEE 802.1p standard has been ready for quite a while. In order to gain full end-to-end QoS all components involved have to implement this standard. Some hudge wendors, like Cisco Systems have not implemented it yet on pure L2 switches. [14]
A big player in desktop workstations software, Microsoft has implemented IEEE 802.1p support only on Windows 98. In future Windows 2000 it will be supported as well. [13] Many other vendors do support IEEE 802.1p in variety of products, it remains uncertain thise two major players will loose their dominance due to this fact.
4.3 Subnet Bandwidth Manager (SBM)
In order to gain end-to-end QoS from ethernet to an other network-technology, we need to glue RSVP and IEEE 802.1p together.
SBM signaling scheme is used to convey 802.1p priorities between layer 2 switches. It will also map class of service between RSVP clients and RSVP-enabled nets.
SBM is a signaling protocol for RSVP-based admission control over IEEE 802 networks, like ethernet. SBM is a method for mapping an internet-level setup protocol such as RSVP onto IEEE 802 networks. In particular, it describes the operation of RSVP-enabled hosts/routers and switches/bridges to support reservation of LAN resources for RSVP-enabled data flows. [15]